1. Field of the Invention
The present invention relates to Doppler speed sensors.
2. Description of the Prior Art
Doppler radar speed sensors for surface vehicles typically use antennas mounted on the side of the vehicle towards the surface across which motion is to take place, such surface hereinafter being referred to as the "surface" or "ground surface". A probe signal is radiated from the antenna toward the surface to impinge on that surface at a predetermined angle of incidence. A portion of the probe signal is reflected from the surface back in the direction from which the probe signal came. Such reflected-back portion of the probe signal is referred to in the art as "backscatter". The remainder of the probe signal is either reflected in other directions or is absorbed by the ground surface. The amount of backscatter from a given surface, is affected by the parameters and radiation pattern of the probe signals, and by the characteristics of the surface. For example, the wavelength, power, and polarization of the radar probe; the area of the surface illuminated by the probe signal; the angle of incidence of the probe signal; permittivity and roughness of the surface all affect the amount and characteristics of backscatter from the surface. The frequency of the backscatter is Doppler-shifted from the frequency of the probe signal (f.sub.o) in accordance with the speed of the relative motion between the vehicle and the surface. The backscatter is received by the radar, and the Doppler frequency shift thereof is determined to derive the relative speed.
Systems, utilizing a plurality of antennas are known. Such a system is described in U.S. Pat. No. 3,833,906, issued Sept. 3, 1974 to C. F. Augustine. The Augustine system comprises two "back-to-back" antennas in a "Janus" type configuration, respectively facing forward and rearward with non-overlapping radiation patterns. Each antenna serves to provide transmitting and receiving functions. The respective backscatter signal portions received by the antennas are added and subtracted to derive sum and difference signals, from which the Doppler speed is determined. "Vehicular Radar Speedometer" by Hyltin, Fuchser, Tyson and Regueiro, International Automotive Engineering Congress and Exposition, Jan. 1973 and "A Lightweight and Self-contained Airborne Navigational System" by Brown, Moody, Thompson, Bibby, Franklin, Ganton, and Mitchell, Proceedings of the IRE, Vol. 47, No. 5, May 1959, pp. 778-807 further describe the prior art. The antennas in Janus type systems typically maintain radiation patterns at the same absolute angle with respect to the underside of the vehicle. Thus, such systems are subject to errors due to relative tilt between the underside of the vehicle and the ground surface, occurring for example when the vehicle goes over a bump in the road.
The prior art systems generally require the antennas to be mounted such that their radiation patterns are in exact accord with a predetermined angle of incidence. To provide for faster and easier antenna installation, it is desirable that a speed sensor be relatively insensitive to angular mounting on the vehicle.